As an example of fluid machines having an expansion mechanism and a compression mechanism, an expander-compressor unit conventionally has been known. FIG. 13 is a vertical cross-sectional view of an expander-compressor unit described in JP 2005-299632 A.
An expander-compressor unit 103 includes a closed casing 120, a compression mechanism 121, a motor 122, and an expansion mechanism 123. A shaft 124 couples the motor 122, the compression mechanism 121, and the expansion mechanism 123. The expansion mechanism 123 recovers power from a working fluid (such as a refrigerant) expanding, and provides the recovered power to the shaft 124. Thereby, the power consumption of the motor 122 for driving the compression mechanism 121 is reduced, and the coefficient of performance of a system using the expander-compressor unit 103 is increased.
The closed casing 120 has a bottom portion 125 utilized as an oil reservoir. An oil pump 126 is provided at a lower end of the shaft 124 in order to pump up an oil held in the bottom portion 125 to an upper part of the closed casing 120. The oil pumped up by the oil pump 126 is supplied to the compression mechanism 121 and the expansion mechanism 123 via an oil supply passage 127 formed in the shaft 124. Thereby, lubrication and sealing are ensured in sliding parts of the compression mechanism 121 and those of the expansion mechanism 123.
An oil return passage 128 is provided at an upper part of the expansion mechanism 123. One end of the oil return passage 128 is connected to the oil supply passage 127 formed in the shaft 124, and the other end thereof opens downwardly below the expansion mechanism 123. Generally, the oil is supplied excessively for ensuring the reliability of the expansion mechanism 123. The excess oil is discharged downwardly below the expansion mechanism 123 via the oil return passage 128.
Usually, the amount of the oil contained in the working fluid is different between the compression mechanism 121 and the expansion mechanism 123. Thus, in the case where the compression mechanism 121 and the expansion mechanism 123 are accommodated in separate closed casings, a means for adjusting the amount of the oil in the two closed casings is essential in order to prevent the amount of the oil from being excess or deficient. In contrast, the expander-compressor unit 103 shown in FIG. 13 intrinsically is free from the problem of the excess or deficient oil amount because the compression mechanism 121 and the expansion mechanism 123 are accommodated in the same closed casing 120.
In the expander-compressor unit 103, the oil pumped up from the bottom portion 125 is heated by the compression mechanism 121 because the oil passes through the compression mechanism 121 having a high temperature. The oil heated by the compression mechanism 121 is heated further by the motor 122 and reaches the expansion mechanism 123. The oil that has reached the expansion mechanism 123 is cooled by the expansion mechanism 123 having a low temperature, and thereafter is discharged downwardly below the expansion mechanism 123 via the oil return passage 128. The oil discharged from the expansion mechanism 123 is heated when passing along a side face of the motor 122. The oil is heated further also when passing along a side face of the compression mechanism 121, and returns to the bottom portion 125 of the closed casing 120.
As described above, the oil circulates between the compression mechanism and the expansion mechanism so that the heat is transferred from the compression mechanism to the expansion mechanism via the oil. This heat transfer lowers the temperature of the working fluid discharged from the compression mechanism and raises the temperature of the working fluid discharged from the expansion mechanism, hindering the increase in the coefficient of performance of the system using the expander-compressor unit.